1. Field of the Invention
This invention relates to a semiconductor device in which low-dielectric-constant insulators are used as interconnecting insulation layers (interlayer insulation films) so as to mitigate any wiring delay of signals (delay in wirings of interconnection) to improve device performance.
2. Description of the Related Art
As semiconductor devices are made to have higher integration and smaller chip size, wirings are being made to scale down in length, to have narrower wiring pitches and to be formed in a larger number of metal layers (i.e., wirings are being made finer, more narrow-pitch and more multiple-layer). With such progress, the delay coming when signals pass through wirings, i.e., the wiring delay tends to increase. This is a problem of great proportions in using electronic equipment making use of semiconductor devices.
In general, the speed of signals which pass through wirings depends on the product (RC) of wiring resistance (R) and wiring-to-wiring capacitance (C). Hence, in order to mitigate the wiring delay, it is necessary to make the wiring-to-wiring capacitance small, i.e., to make the interconnecting insulation layer have a low dielectric constant.
As measures to lower the wiring resistance, it is set forward in high-performance semiconductor devices to change their wiring material from aluminum to copper. In particular, the damascene structure where copper wirings are buried in interconnecting insulation layers is actively applied in processing.
As measures to make the interconnecting insulation layer have a low dielectric constant, inorganic materials such as a silicon oxide film (SiO2: dielectric constant about 4.0) and a silicon nitoride film (SiN: dielectric constant about 7.0) formed by CVD (chemical vapor deposition) have conventionally been used in interconnecting insulation layers of semiconductor devices. Then, recently, these are succeeded by employment of a fluorine-dopped silicon oxide film (SiOF: dielectric constant about 3.6) as a material with low dielectric constant that can continue conventional processes.
However, the fluorine-doped silicon oxide film has a relatively high dielectric constant, and can not have a sufficient effect of lessening the wiring-to-wiring capacitance when it is used as the interconnecting insulation layer. Accordingly, in semiconductor devices since the generation of wiring process of 90 nm nodes, materials having much lower dielectric constant are required.
As materials of interconnecting insulation layers having a property that the dielectric constant is lower than 3.5, various materials are proposed. In rough classification, studies are made on what is called spin-on-glass materials with which substrates are coated followed by heating to form films, on organic materials similarly formed into films, and on methods of forming films by CVD.
As the spin-on-glass materials, there are materials containing a hydrogen silsesquioxane compound, a methyl silsesquioxane compound, and the like. The materials composed chiefly of a hydrogen silsesquioxane compound or a methyl silsesquioxane compound are preferred. In the present specification, a chief ingredient is a compounent of the hightest combination ratio (a mole ratio).
A coating solution composed chiefly of the hydrogen silsesquioxane compound is one prepared by dissolving the compound, which is represented by the general formula: (HSiO3/2)n, in a solvent such as methyl isobutyl ketone. A substrate is coated with this solution, which is then subjected to intermediate heating at a temperature of approximately from 100° C. to 250° C., followed by heating at a temperature of from 350° C. to 450° C. in an inert atmosphere, e.g., in an atmosphere of nitrogen, so that an insulation layer is formed in which Si-O-Si bond networks are formed in ladder structure and which is finally chiefly composed of SiO.
A coating solution composed chiefly of the methyl silsesquioxane compound is one prepared by dissolving the compound, which is represented by the general formula: (CH3SiO3/2)n, in a solvent such as methyl isobutyl ketone. A substrate is coated with this solution, which is then subjected to intermediate heating at a temperature of approximately from 100° C. to 250° C., followed by heating at a temperature of from 350° C. to 450° C. in an inert atmosphere, e.g., in an atmosphere of nitrogen, so that an insulation layer is formed in which Si—O—Si bond networks are formed in ladder structure and which is finally chiefly composed of SiO.
As organic insulation layer materials, polymeric materials such as polyimide, poly(p-xylylene), poly(arylene) ether, poly(arylene), benzcyclobutene and polynaphthalene, which are hydrocarbon type resins, are known in the art. These materials contain carbon atoms, in virtue of which the film is made to have a low density, and also the polarizability of molecules (monomers) themselves is made small, in virtue of which the film achieves a low dielectric constant.
As methods of more reducing the dielectric constant of interconnecting insulation layers such as the above spin-on-glass films, organic films and CVD films, it is known to form nano-pores in films to make the films into porous films. With regard to the above materials and processes, they are disclosed in International Technology Roadmap for Semiconductors, 1999 Edition, pp. 163-186, and Japanese Patent Applications Laid-open No. 2000-340569 and No. 2001-274239.
However, in the above related art, the interconnecting insulation layers having the property that the dielectric constant is lower than 3.5 involve a problem that the insulation layers have fundamentally lower mechanical strength such as hardness and elastic modulus than the Silicon oxide film and Silicon nitride film formed by CVD.
In such insulation layers, it has been considered not realistic that the nano-pores are formed in films to make the films into porous films in order to more reduce the dielectric constant, because this may come toward further deterioration of mechanical strength.
As a means for lowering the dielectric constant of insulation layers, insulating organic polymers such as polyimide are used in some cases. Such organic polymers are favorable because their dielectric constant is less than 4, but have disadvantages that they physically have a lower mechanical strength and also higher hygroscopicity and moisture permeability than inorganic films. When used as interconnecting insulation layers, they may also cause a problem on the reliability of devices, e.g., a lowering of mechanical strength of device structure and corrosion of wirings which is due to absorbed moisture.